Transformers in which grain-oriented electrical steel sheets are used are required to have low iron loss and low noise properties. In order for a transformer to have lower iron loss, it is effective to reduce the iron loss of the grain-oriented electrical steel sheet itself, and as one of the techniques for doing so, it is necessary to irradiate the surface of the steel sheet with laser beams, plasma, electron beams, or the like. JP2012036450A (PTL 1) teaches a technique for reducing iron loss by optimizing the interval between irradiation points and irradiation energy when introducing thermal strain to a surface of a grain-oriented electrical steel sheet in a dot-sequence manner by electron beam irradiation in a direction transverse to a rolling direction. This technique reduces iron loss by not only refining main magnetic domains but also forming an additional magnetic domain structure, called closure domains, inside the steel sheet.
As closure domains increase, however, this technique has a disadvantage in noise performance when incorporated in a transformer. The reason is that since the magnetic moment of closure domains is oriented in a plane orthogonal to the rolling direction, magnetostriction occurs as the orientation changes towards the rolling direction during the excitation process of the grain-oriented electrical steel sheet. The steel sheet also contains other closure domains called “lancet domains”, and magnetostriction also occurs as a result of generation and disappearance of such lancet domains during excitation with alternating magnetic fields. It is known that lancet domains can be reduced by applying tension, for example, and the reduction of lancet domains can yield improved magnetostriction properties. On the other hand, closure domains caused by magnetic domain refinement as described above also cause magnetostriction and deterioration of transformer noise performance. Therefore, there is demand for optimization of not only lancet domains but also closure domains in order to achieve both low iron loss and low noise properties.
Conventional techniques for improving iron loss properties and noise performance with electron beam methods are described below. JP2012172191A (PTL 2) teaches a technique for providing a grain-oriented electrical steel sheet exhibiting excellent iron loss properties and noise performance by adjusting, in the case of performing magnetic domain refining treatment by irradiating with an electron beam in point form, the relationship between holding time t at each irradiation point and interval X between irradiation points in accordance with the output of the electron beam. JP2012036445A (PTL 3) describes a grain-oriented electrical steel sheet in which magnetic domain refining treatment is performed with electron beam irradiation and the relationship between diameter A of a thermal strain introduction region and irradiation pitch B is optimized.
WO2014068962A (PTL 4) describes a technique for optimizing, using an electron beam method, the rolling-direction width and the thickness-direction depth of closure domains as well as the interval at which closure domains are introduced in the rolling direction.